Gas Transport (series I)

•O2 picked up by the blood at the lungs must be transported to the tissues for cell use & CO2 produced at the cellular level must be transported to the lungs for elimination OBJECTIVES OF GAS TRANSPORT •O2 transport (O2 carriage) in blood: O2 carrying capacity; O2 content of blood; oxyhaemoglobin dissociation curve •O2 delivery & types of hypoxia •Carbon monoxide poisoning •Transport of CO2 in blood: CO2 dissociation curve; Haldane effect

O2 is transported in blood in two different forms: (I) as dissolved O2 (~ 1- 2 %) (ii) O2 bound to Hb not RBCs (~ 98 – 99 % of total O2 content in blood)

• • • •

The amount of dissolved O2 is directly proportional to PO2 (Henry’s law) Amount of dissolved O2= PO2 X 0.003 ml O2/dl blood solubility coefficient for O2 (At 37oC, each mm Hg PO2 results in 0.003 ml O2 dissolved in 100 ml of blood)

(2) O2 bound to Hb + O2 •Hb DEOXYHAEMOGLOBIN

HbO2 OXYHAEMOGLOBIN

•Each Hb molecule can bind maximally with 4 O2 molecules •When Hb is 100% saturated with O2, 1g Hb binds 1.34 ml O2 •The maximum amount of O2 that can be bound to Hbper volume of blood (ie. when all the Hb present in a volume of blood are fully saturated with O2) is known as the oxygen carrying capacity of blood / O2-binding capacity of blood / O2 capacity of blood = [Hb] x 1.34 eg,. If [Hb] is 10g/dl, the O2 carrying capacity of blood is 13.4 ml O2 /dl of blood or 134 ml O2 / litre of blood

Arterial blood O2 content (CaO2) (ml O2/dl blood) = (amount of O2 bound to Hb) + (dissolved O2) = ([Hb] x 1.34 x % Hb saturation) + (PO2 x 0.003) In normal adult male at rest: PaO2 100 mm Hg, [Hb] is 15g/dl CaO2= (15 x 1.34 x 97/100) + (100 x 0.003) = 19.8 ml O2/100 ml blood

 ; O2 saturation of Hb (SO2) O2 that binds to Hb X 100 Maximum amount of O2 that binds to Hb - reflects amount of O2 that bind to Hb

Most impt factor determining the Hb satn is PO2 of blood Relationship shown thro’ graph called oxyhaemoglobin dissociation curve or O2-haemoglobin

PULSE OXYMETER – MEASURE HB SATURATION OF PATIENTS

Oxyhaemoglobin dissociation curve of normal adult arterial blood (PCO2 40 mmHg, pH 7.4, temp. 37oC, normal [2,3-diphosphoglycerate] or 2,3-DPG / 2,3bisphosphoglycerate or 2,3-BPG in RBCs; HbA)

Sigmoid shape

At PO2 100 mmHg, Hb nearly saturated with O2

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(mm Hg)

Oxyhaemoglobin dissociation curve of normal adult arterial blood (PCO2 40 mmHg, pH 7.4, temp. 37oC, normal [2,3-diphosphoglycerate] or 2,3-DPG / 2,3bisphosphoglycerate or 2,3-BPG in RBCs; HbA)

At PO2 100 mmHg, Hb nearly saturated with O2

Sigmoid shape

-

(mm Hg)

Oxyhaemoglobin dissociation curve of normal adult arterial blood at PaCO2 40 mm Hg, pH 7.4, temp. 37oC, normal [2,3-DPG; 2,3-diphosphoglycerate / 2.3bisphosphoglycerate] in RBCs

At PO2 100 mm

Hg, Hb nearly saturated with O2

P50 – PO2 where the Hb saturation is 50% (27 mm Hg)

(mm Hg)

Oxyhaemoglobin-dissociation curve

Physiological significance of the sigmoid shape of the O2-Hb dissociation curve      

(ii) Flat upper part or plateau portion of the curve PO2can drop to ~ 75 mm Hg (due to pulmonary disease, high altitude etc) but there is little reduction in % Hb saturation & therefore the O2 content of blood

Oxyhaemoglobin dissociation curve of normal adult arterial blood at PaCO2 40 mm Hg, pH 7.4, temp. 37oC, normal [2,3-DPG; 2,3-diphosphoglycerate / 2.3bisphosphoglycerate] in RBCs

(mm Hg)

Arterial blood O2 content (CaO2) = ([Hb] x 1.34 x % Hb saturation) + (PO2 x 0.003) If [Hb] is 15g/dl & PaO2 100 mm Hg, CaO2= (15 x 1.34 x 97/100) + (100 x 0.003) = 19.8 ml O2/100 ml blood At PaO2 80 mm Hg, Hb saturation = 96% CaO2 = 19.5 ml O2/100 ml blood  

PaO2 CaO2

Normal person hyperventilates, PaO2 ~ 160 mmHg, CaO2 does not ↑ much because Hb is nearly saturated with O2 at PaO2 100 mm Hg (contribution of dissolved O2is small)

Factors affecting the Hb affinity for O2 & the oxyhaemoglobin dissociation curve Normal arterial blood oxyHb dissociation curve (PCO2 40 mmHg, pH 7.4, temp. 37oC, HbA) (> 37oC) (> 40 mm Hg) (in RBC) (<7.4)/acidosis

P5

P50

↓ affinity of Hb for O2 OxyHb dissociation curve shifts to the right P50 ↑ (> 27 mm Hg) (an index of Hb affinity for O2) - facilitates O2 unloading as the blood passes through the tissue capillaries

Exercising muscle or other actively metabolising cel produces more CO2, [H+] & heat production → ↓ affinity of Hb for O2 (oxyHb dissociation curve shifts to the right) → enhances O2 release from Hbfor use by the active tissues

Factors affecting the Hb affinity to O2 & the oxyhaemoglobin dissociation curve (< 37oC) < 40 mm Hg) (alkalosis)

HbF

P50

Normal arterial blood

Curve shifted to the left ↑ affinity of Hb to O2 (Hb more saturated at a given PO2) P50 ↓ Leftward shift – Hb release relatively less O2 at tissue level but pick up more O2 at pulmonary capillary level

when PaO2 is within normal limits, a shift of the oxygen dissociation curve to the right or left usually does not significantly affect the Hb’s ability to transport O2 to the peripheral tissues

Factors affecting the Hb affinity for O2 & the oxyhaemoglobin dissociation curve Normal arterial blood oxyHb dissociation curve (PCO2 40 mmHg, pH 7.4, temp. 37oC, HbA) (> 37oC) (> 40 mm Hg) (in RBC) (<7.4)/acidosis

P5

P50

↓ affinity of Hb for O2 OxyHb dissociation curve shifts to the right P50 ↑ (> 27 mm Hg) (an index of Hb affinity for O2) - facilitates O2 unloading as the blood passes through the tissue capillaries

Some factors that        

CaO2 ↓ in anaemia due to low [Hb] Compensatory processes in chronic anaemia to improve O2 delivery to tissues:  [2,3-DPG] (due to chronic hypoxia)  HbO2 curve shifts to the right - facilitates O2 release from Hb to the tissues 6 ↑ CO → ↑ blood flow to the tissues

Some factors that         

2 factors determine rate of O2 delivery to tissues (how much O2 delivered to tissues per min. (ml O2/min.)) (i) CaO2 (O2 content of arterial blood) (ii) Q (blood flow (ml/min) or CO Rate of O2delivery to the tissues = CaO2 X Q    

Rate of O2delivery to the tissues = CaO2 X Q CaO2 = ([Hb] X 1.34 X % Hb satn) + (PO2 X 0.003)

 

Rate of O2delivery to the tissues = CaO2 X Q CaO2 = ([Hb] X 1.34 X % Hb satn) + (PO2 X 0.003)

   •

most common

§         

 



Oxyhaemoglobin dissociation curve of normal adult arterial blood at PaCO2 40 mm Hg, pH 7.4, temp. 37oC, normal [2,3-DPG; 2,3diphosphoglycerate / 2.3-bisphosphoglycerate] in RBCs

(mm Hg)

Mechanisms or physiological causes of hypoxaemia (→  •

 

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  

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  

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

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

↓↓ PaO2 → more [deoxyHb] in blood When[deoxyhaemoglobin] > 5g/dl capillary blood →  t – seen in lips, tongue, nailbeds